1. Field of the Invention
The invention concerns a permanently magnetized bar including magnets disposed between polepieces.
2. Description of the Prior Art
Many magnetic devices, such as magnetic bearings in particular, use permanent magnets inside permanently magnetized bars which are either straight or curved to form rings.
The permanently magnetized rings are used in the devices described in U.S. Pat. Nos. 3,937,533, 3,955,858, 4,000,929, 4,043,614, 4,077,678, 4,211,452, 4,470,644 and European Patent 0,284,487.
The rings include permanently magnetized cores between two ferromagnetic polepieces. If the direction of permanent magnetization is axial, the two polepieces are on opposite axial sides of the core and are generally flattened transversely to the axis: the two polepieces are in practice identical and have mean diameters substantially equal to the mean diameter of the core. If the direction of permanent magnetization is radial, the two polepieces are concentric cylinder portions on opposite radial sides of the core.
European Patent No. 0,284,487 also describes a straight magnetic bearing using straight permanently magnetized bars. In this case both the permanently magnetized core and the polepieces on either side of it are straight, the two polepieces being disposed transversely to the direction of permanent magnetization.
The cores are designed to generate as constant as possible a magnetic flux along elongate projections of the bar designed to border the airgaps, in practice edges of the polepieces, whether the bar is curved or straight. This is why the core is in practice made entirely from a permanently magnetized material such as Samarium-Cobalt.
There are problems in making continuous bars of permanently magnetized material as only small straight bars or rings can be made in one piece given the requirements for homogeneous magnetization and compacting during sintering. For larger sizes (typically diameters exceeding 70 mm) use is made of elements usually called "tiles" which are designed to fit together edge-to-edge with maximum precision to approximate a continuous magnetized core. Especially if the core to be made is curved (usually a ring) its fabrication is time-consuming and costly given the obligation to make the tiles with a very accurate geometry specific to the bar to be made and to give all the tiles the same magnetization characteristics. The tiles cannot be manufactured until the design of the ring is entirely finished.
An object of the invention is to overcome these drawbacks by means of a permanently magnetized bar having a permanently magnetized core between polepieces and with a structure that is easy to manufacture at lower cost than previously with short manufacturing times, with the magnetic flux along the polepieces, in practice along their edges forming elongate projections adapted to border the airgaps, guaranteed to be at least as homogeneous as in the past.